One thing to consider would be what support for which codecs would be included.

The MPlayer project takes an activist perspective on patents and IP - namely, they openly disregard these because they are ideologically opposed to the restrictions these attempt to impose. Xine and VLC take similar standpoints, and have got away with it so far. More power to them I say.

Not all linux distros are prepared to take that chance however. Debian will not distribute these programs because their legal team cannot agree on the risk profile, though it is probably quite low if M$ codecs are not included. However iirc lame support for encoding, libdvdcss decode, dts decode, faac and some others are controversial and in theory might incur license fees.

If I remember correctly, mplayer needs to be specifically compiled to check the processor in order to be usable on the greatest number of computers, but maybe it can be set to work only with x86 (I think i586 leaves out some older machines supported by DSL).

Also don't see *much* use in a gui for mplayer, although that's just opinion. Even back when I did include the gui in my installs, I found that the only time I ever used it was for file browsing. Mplayer has a very good and easy keyboard control for just about anything related to playback.